Predatory birds called black kites have an unusual
decorating style: They line their nests with bits of white plastic. Now, a new
study suggests that this lining of litter may warn other birds that the owner
of the nest is ready to fight.

Black kites are medium-sized raptors found across Europe and
Asia. Males and females build nests together, lining the nests with white plastic
right before the female lays her eggs. Before humans started leaving
bits of trash around, the birds may have used feathers from other birds,
said study researcher Fabrizio Sergio, a researcher at the department of
conservation biology at the Consejo Superior de Investigaciones Científicas in
Seville, Spain.

Or the behavior "could have even evolved after mankind
came about," Sergio told LiveScience. "We really don't know."

Decorating with white

The researchers also weren't sure what the birds want with
the white plastic. Other bird species and some fish use decorative objects or
structures to attract mates. The Australian bowerbird male even builds a structure
complete with an optical illusion to make
himself look larger to potential mates. But kites decorate after they've
paired up.

To unravel the mystery of the white plastic, Sergio and his
colleagues observed a well studied, banded population of kites in Doñana
National Park in Spain. The birds have been under observation for decades, and
researchers have records on the ages of most of them.

First, the researchers recorded the level of decoration in
127 nests built by the raptors, a highly territorial species. They found that
77 percent of the nests had some decoration, which the birds began to add in
the 20 days before laying eggs. Both males and females went plastic-scavenging.
When the researchers provided a variety of colored plastics to choose from, the
birds almost always turned up their beaks at every color except white.

Birds at their reproductive peak were the most likely to
decorate their nests with lots of plastic. When the researchers offered up
extra plastic for birds that wanted it, 37 percent of the mated pairs refrained
from collecting the plastic at all. These birds were either very old, very
young, or in possession of poor-quality territory, Sergio said. [Image of elaborately decorated nest]

Kites sometimes vie for territory in mid-air fights, slashing
each other with their sharp talons. These battles aren't for the weak, Sergio
said.

Decoration-loving birds were more successful at fending off
intruders and experienced fewer attacks overall, the researchers noticed. The
same birds dominated when vying for food away from the nest, and their
offspring had higher survival rates.

The researchers suspected that the birds might be using the
plastic to make a statement about their high-quality territory and their
ability to defend it – the bird equivalent of a "No Trespassing" sign
backed up by the sound of someone loading a shotgun. To test the idea, Sergio
and his team added extra white plastic to the birds' nests. They found that
attacks initially increased, as if they'd painted a target on the nests.
Dominant birds were usually able to fight off the attacks, but weaker birds
weren't. In fact, they seemed desperate not to draw attention to themselves.

"We saw that some individuals were actually removing
the decorations, and once again it was the very young and the very old,"
Sergio said.

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In a final experiment, the research team created fake nests
stocked with chicken
eggs. Some of the nests were lined with plastic, while others were
plastic-free. Again consistent with the interpretation that plastic is a sign
of bird wealth, the plastic-lined nests were burglarized and the eggs destroyed
in 81 percent of cases compared with about 31 percent of plastic-free nests.

Sergio said the data point to white decorations as a
territorial signal. When other birds see a sudden jump in white plastic, they
test the truth of the signal: Is the bird bluffing? Can it really defend its
plum location? If the nest-owners successfully show off their strength and
defeat the intruders, the nest will be left alone.

Alternative explanation

Not everyone is convinced, however. The experiments were
interesting, but the data could support another story, said Gerald Borgia, a
professor of biology at the University of Maryland who was not involved in the
study.

Instead of drawing attention to the nests with the white
plastic, Borgia suspects the birds are trying to camouflage
their white eggs.

"Eggs are vital things that parents want to take care
of," Borgia told LiveScience. "To put a signal under eggs that would
make them more visible, that seems very strange."

Instead, kites may use the white plastic to make it more
difficult for aerial predators to see the eggs. Younger, older and weaker birds
may have less plastic because dominant birds are better able to collect plastic
or steal it from weaker birds, Borgia said.

Under this explanation, the increase in attacks on
white-lined dummy nests would be because of kites swooping in to steal the
unattended white plastic, Borgia said.

The researchers didn't notice any thieving, but observations
were limited to in-person visits rather than constant video monitoring, Borgia
said: "They need to do some kind of intensive monitoring to determine if
there is stealing."

Sergio said the alternative explanation is "very
unlikely," noting that hundreds of hours of observation had turned up no
instances of thievery. Furthermore, he said, the birds that decorate their
nests always lay eggs, while those that don't sometimes lay eggs and sometimes
fail to breed. That makes the white plastic a bad camouflage strategy, Sergio
said.

"Instead of concealing them, the birds that decorate
could be giving away the information that there are eggs or there will soon
be," Sergio said.

Sergio and his team plan to continue their observations of
the kites and the birds' nest-decorating techniques.

Stephanie Pappas

Stephanie Pappas is a contributing writer for Live Science. She covers the world of human and animal behavior, as well as paleontology and other science topics. Stephanie has a Bachelor of Arts in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz. She has ducked under a glacier in Switzerland and poked hot lava with a stick in Hawaii. Stephanie hails from East Tennessee, the global center for salamander diversity. Follow Stephanie on Google+.